Hysteresis in Asphaltene Precipitation and Redissolution

Abstract

The precipitation and redissolution of asphaltenes from mixtures of Athabasca bitumen and n-heptane was measured over time in both air and nitrogen atmospheres at 23 °C. In air, it appears that oxidation of the bitumen increased the asphaltene yield for as long as the experiments were conducted; that is, for several months. When oxidation effects are excluded, asphaltene precipitation and redissolution both appear to reach steady state within 24 h. A hysteresis between asphaltene precipitation and redissolution was also observed in both atmospheres. The hysteresis was exaggerated in the air atmosphere due to oxidation effects. In both air and nitrogen atmospheres, complete reversibility was attained when the heptane-to-bitumen ratio was reduced to 1.5 cm3/g. The hysteresis is attributed to an energy barrier to asphaltene dissociation. As more heptane is added, the asphaltenes likely self-associate to higher apparent molar masses. When the heptane is removed, the asphaltenes may remain in the higher association state and hence precipitate to a greater extent than before at a given heptane-to-bitumen ratio. If the system is heated, the asphaltene yields return to their original values, indicating that the original association state can be restored.